Roger Mills writes:
Does anyone have any insight as to what can happen to a micro-SD card
such that it can only be read incredibly slowly, seemingly cannot be
written to at all (and, no, it's not write protected) and makes a
phone think that it's been removed when it hasn't?

Its broken. Chuck it in the bin.

Is there any possibility that using FTP between computer and phone
could have damaged the card in some way?

No.

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http://www.greenend.org.uk/rjk/